This invention relates to an improved lime-fly ash-aggregate (LFA) pozzolanic composition which is useful as a load-carrying base for roads, airport runways, parking lots and the like.
As the cost of raw materials has increased, greater interest in utilization of waste materials as a means of providing cost benefits has also increased. This interest in waste utilization has also been spurred by the increasing cost and difficulty of disposal of waste materials in view of increasingly stringent pollution standards. In addition, utilization of waste materials also helps to conserve natural resources.
An outstanding example of utilization of waste material is the use of fly ash from the combustion of powdered coal, recovered from power plant flue gases by mechanical and/or electrostatic precipitation. One economical use of fly ash has been its use in LFA mixtures.
In order to obtain maximum integrity and stability in modern highway construction, a series of layers is built-up. The construction normally comprises a subgrade or road bed, a sub-base, a base course and a wearing course. The sub-base provides uniform strength, which allows the base course to be a uniform and reasonable thickness. The base course furnishes nonyielding support to the wearing course, and helps to transfer and distribute wheel load to the subgrade. The wearing course, which can be concrete or asphalt, protects the lower layers from traffic load, from weathering and from water infiltration.
Because of the importance of the support provided by the base course, and because of the large quantities of materials used in highway construction, engineers are interested in economical materials which possess the desired strength. One economical material widely used involves a technology used by the Romans and others many years ago, the use of pozzolanic materials. While the Romans used natural pozzolans such as volcanic ash or shale, modern pozzolans are generally fly ash.
A pozzolan can be defined as a material which reacts with lime in the presence of moisture to form a cementitious material, although the pozzolan possesses little or no cementitious properties in itself. Although the chemistry of the lime-pozzolan reaction is complicated and not completely understood at present, it is believed that the silica and alumina in the fly ash react with the calcium oxide or calcium hydroxide in the lime to form calcium silicate and calcium aluminate gels, along with other complex compounds involving alkalis, iron and aluminum. Sufficient water must be present for the reaction to occur. The rate of reaction decreases with decreased temperatures and is retarded to a great extent at temperatures lower than 40.degree. F. The presence of large amounts of organic materials may be deleterious to the pozzolanic reaction.
As previously referred to, disposal of waste products from industrial processes presents a challenge to more efficient use of natural resources. Large amounts of waste products are produced by the treatment of domestic and industrial water which involves concentration of pollutants from raw water into a smaller volume and subsequent separation from the treated water by means of inorganic and/or organic coagulants and precipitating agents for clarifying and/or softening the water. The resulting solids-water suspension is referred to as a "sludge".
Heretofore, a common practice in disposing of sludge was discharge of the sludge to surface water, although a small amount of sludge has been disposed of by recalcination. The 1972 Water Pollution Control Act amendments established federal standards which provide industrial discharge guidelines. These guidelines call for the use of best practical control technology by July, 1977, best available control technology by July, 1983, and elimination of pollutant discharges by 1985.
Water softening or the removal of calcium and magnesium ions from water can be accomplished by treatment with lime or lime-soda ash. Addition of lime to water which contains carbonates or bicarbonates causes precipitation of CaCO.sub.3 and Mg(OH).sub.2. Generally, the most significant portion of the lime sludge produced is calcium carbonate, together with smaller amounts of silicates, organic matter, clay or salt, and hydoxides of iron, magnesium, manganese and aluminum, depending on the ratio of calcium to magnesium in the water, the type and amount of other matter in the water prior to treatment, and whether or not other inorganic or organic coagulants are used concurrently to increase the sludge settling or dewatering efficiency. Inorganic coagulants include alum, aluminum sulfate, ferric sulfate, ferric chloride, ferrous sulfate, ferrous chloride, colloidal activated silica or bentonite. Organic coagulants include cationic, anionic or nonionic organic polyelectrolytes which are available commercially.
The instant invention discloses that incorporation of water treatment sludge into an LFA mixture increases compressive strength and leachability resistance of the mixture.